Thank you for writing. It’s possible that part of what is going on between you and your dad is generational. When he was 18, he probably didn’t feel he had the choice to be floundering. He just had to work. It’s hard for him to understand that you are more typical of your age group than not. It’s an unusual 18-year-old these days who is certain about what they want to do with their life.
The other issue that stands between you is that your dad is worried about you. He may not express it in the best way but I suspect he just wants to know that you’re going to be okay. It’s an occupational hazard for parents.
My suggestion is this: Do something. Almost anything. Any job you take will help you figure out what you do and don’t want to do with your life. You may find elements in the job that feel right and you may not. But you will get more information. Then try something else. As you gain experience with different options, it will start to become clear where your interests and talents lie. And don’t tell me you’re not motivated. You don’t have to be. All you have to do is get started. Sitting in your confusion isn’t going to give you any new information. Trying some things out will.
Once you get a job, let your dad know that you are as concerned as he is and that you are taking steps to figure it out. Tell him that his support in the project means a great deal to you. Invite him to talk with you about what you learn about each job. Once he sees you actively working on taking care of yourself, he’ll be more able to be supportive.
I wish you well.